When are "Allah is your Lord" and "The Most Gracious is your Lord" used?

The phrase "ربكم الله" (your Lord is Allah) appears twice in the Holy Quran: in verse 54 of Surat Al-A'raf and verse 3 of Surat Yunus. As for the phrase "ربكم الرحمن" (your Lord is the Most Merciful), it appears once in verse 90 of Surat Taha. Harun's (peace be upon him) use of the name "Al-Rahman" might have been intended to win over the Children of Israel to repentance and remind them of Allah's mercy and forgiveness, or because he was inherently merciful.